The present invention relates to a vehicle transmission, and more particularly to a reverse gear mechanism for use in a vehicle gear transmission.
There are known vehicle gear transmissions having two shafts (primary and secondary shafts or main and counter shafts) and a plurality of gears interposed in meshing relation between these two gears. Reverse gear mechanisms including idle gears are widely employed in such known vehicle gear transmissions.
Power transmission units for use on vehicles include various parts involved in torque transmission, such as a propeller shaft, wheel axles, etc., in addition to the shafts in the gear transmissions. The torque transmitted by one of such parts is defined by the speed reduction ratio from the engine to that component (the ratio of the r.p.m. of the component to the r.p.m. of the engine). The possible maximum torque, which is one of the most important factors for determining the shaft diameters, therefore varies dependent on the speed reduction ratio. In a vehicle power transmission unit including a gear transmission having a primary shaft connected to an engine and a secondary shaft connected to a drive wheel, for example, the torque that can be transmitted by the parts from the secondary shaft to the wheel axle inclusive varies in proportion to the speed reduction ratio of a selected gear of the gear transmission.
In general, the speed reduction ratio of the reverse gear is selected to be larger than that of the low gear since the driver normally wishes to move the vehicle at a lower speed when it is to be moved back. Therefore, the possible maximum torque transmitted by the foregoing parts such as the secondary shaft, etc. is largest when the reverse gear is selected. Heretofore, the dimensions, such as shaft diameters, of the parts in the power transmission unit have been determined in view of the speed reduction ratios from the engine to the parts at the time the vehicle is to be moved back. However, it would seldom happen for the drive wheel to require a large torque when the vehicle is moving back. Large torques are required more frequently when the low gear is selected. An increase in the diameter of a shaft involved in power transmission results directly in an increase in the rotational inertial mass of the shaft, thereby making the fuel economy of the vehicle poor and requiring the brake unit to be large in size. In view of these difficulties, the conventional gear transmission which requires the shaft diameters to be determined as described above still remains to be improved.